(1) Field of the Invention
The present invention relates to a wet oxidizing process for a waste liquid, the so-called waste soda. When light hydrocarbons containing sulfur components such as naphtha, butane and ethane are thermally cracked, sulfur compounds are contained in the obtained hydrocarbon fluid of ethylene and propylene. The sulfur content of this hydrocarbon fluid is removed by absorption using an aqueous alkali and the aqueous alkali after the absorption is discharged as the waste soda.
(2) Description of Prior Art
When hydrocarbons containing sulfur components are processed by thermal cracking, catalytic cracking or catalytic reforming, the sulfur components are mainly converted into hydrogen sulfide. Because the hydrogen sulfide has undesirable actions such as catalyst poison in the subsequent purifying step of the cracked products, it is required to remove such hydrogen sulfide.
As an industrial removing method, a method for absorbing and removing by bringing the hydrogen sulfide into contact with an aqueous alkali has been commonly employed. The aqueous alkali waste which is discharged from the washing operation by the use of an aqueous alkali is usually called as "waste soda", which has strong offensive smell due to the absorbed hydrogen sulfide and which exhibits also a high value of chemical oxygen demand (COD). That is, the simple absorption of the sulfur components with an aqueous alkali still involves such a problem that when the absorption liquid is acidified due to some change in environmental conditions, the hydrogen sulfide is released from the absorbed liquid. In view of the antipollution, it is necessary to take effective measures for the removal of bad smell and decreasing of COD value.
Among the methods or processes which have been proposed heretofore as the operations for excluding noxious effects of the above described aqueous alkali waste, a process for oxidizing hydrogen sulfide by bringing the same into contact with molecular oxygen under the condition in which water can exist in liquid phase, i.e., a wet oxidizing process is considered as a desirable process in industrial point of view. This process is the one for oxidizing the sulfur atom of hydrogen sulfide to its higher oxidized state, so that even when the absorption liquid is acidified, hydrogen sulfide is not liberated.
However, when the sulfur atom reaches a higher oxidized state, there is a possibility at the same time that the pH of waste liquid after the oxidation is lowered, and it results in the apprehension for the corrosion of oxidation reactor in such a low pH.
As the countermeasures against the above described corrosion, a variety of methods or processes have been proposed. For instance, it is proposed in Japanese Patent Laid-Open Publication No. 4-338285 that various materials contained in a waste soda liquid are analyzed to determine the amount of alkali required for neutralizing the sulfur produced by oxidation on the basis of primary and secondary dissociation constants of hydrogen sulfide and carbonic acid by means of calculation, and the amount of alkali additionally required for the neutralization is added to the solution to be treated.
More specifically, in the above method as disclosed in the Laid-Open patent gazette, the total alkalinity, total sulfides, mercaptans, COD, thiosulfates, total carbonates, and pH have previously been measured by means of analysis of waste soda, the amount of total sulfides is separated into the amount of sulfides and that of hydrosulfides, while the amount of total carbonates are separated into the amount of carbonates and that of bicarbonates by calculation on the basis of the primary and the secondary dissociation constants of hydrogen sulfide and carbonic acid obtained from a literature, and the amount of existing caustic soda is calculated from the pH value to estimate the quantity of alkali consumed in the process of the wet oxidizing reaction, whereby an alkali in the amount to be additionally consumed in the reaction is added to the aqueous alkali waste. This method is the one for calculating the amount of hydrosulfide as the source for generating acids in the process of wet oxidation, thereby determining the minimum amount of alkali required for avoiding the change waste liquid into acidic with the generated acids.
However, because the method adopts a calculating method in which dissociation constants of hydrogen sulfide and carbonic acid are used as the basis of determination of alkalinity and quantitative determination of hydrogen sulfide, the reliability in confirming the safety level depends upon the precision of these dissociation constants. It is generally well known that several different numerical values have been reported for the dissociation constants of a variety of materials. In this respect, because the estimation on the safety in operations depends upon the dissociation constants adopted, the uncertainty cannot be avoided. Furthermore, since these dissociation constants are functions of temperature and pressure, the level of certainty is lowered unless these points are also taken into consideration.
The technique described in Japanese Patent Laid-Open Publication No. 7-979 is also the one wherein wet oxidation is conducted by adopting a calculating method using also dissociation constants of hydrogen sulfide and carbonic acid as in the above described method, and this method involves the similar problems as those of the former method.
Moreover, in a treating process of waste soda by means of wet oxidation, it is also important to increase an efficiency for lowering COD in addition to the importance in the above-mentioned estimation of the prevention of corrosion due to the elevation of acidity in the fluid to be processed.
That is, the sulfur content absorbed in the aqueous alkali exists in an equilibrium condition as sodium sulfa and sodium hydrosulfide in the waste soda employed in the present invention. When both of the sulfate and the hydrosulfide are subjected to wet oxidization without the presence of any catalyst, the reactions of two stages proceed through sodium thiosulfate, respectively, as described hereunder:
(1) Oxidation of sodium sulfide
2S.sup.---- +2O.sub.2 +H.sub.2 O.fwdarw.S.sub.2 O.sub.3.sup.---- +2OH.sup.--
S.sub.2 O.sub.3.sup.---- +2O.sub.2 +2OH.sup.--.fwdarw.2SO.sub.4.sup.---- +H.sub.2 O
(2) Oxidation of sodium hydrosulfide
2SH.sup.-- +2O.sub.2.fwdarw.S.sub.2 O.sub.3.sup.---- +H.sub.2 O
S.sub.2 O.sub.3.sup.---- +2O.sub.2 +2OH.sup.--.fwdarw.2SO.sub.4.sup.---- +H.sub.2 O
As shown in the above equations, in order to proceed rapidly the oxidation of sodium thiosulfate being in the intermediate step of the oxidation reaction, it is necessary that the OH.sup.-- ions exist in the reaction system.
Concerning the type of reaction in the industrial working, it is apparent that a continuous operation is more preferable than a batch operation, and in the case where the continuous operation is adopted, it is required particularly to proceed the reaction within a limited period of time, so that the higher rate of reaction is advantageous. Accordingly, it is important to check up whether the composition of the waste soda is in a suitable condition for the rapid reaction or not.